Imagine, if you will, the process of specifying an industrial dryer if you were explaining it in a classic children’s book.
John has a problem. He has a product that is too wet, so John needs a dryer. John does research. John finds that the style of dryer with the largest capacity for the most affordable price is a direct-fired, rotary-drum dryer. John calls an industrial dryer manufacturer, Rotary-Drum Dryer One (RDD1).
RDD1 asks John what product he is processing. It just happens to be a granular-solid mineral. This is great because RDD1 specializes in drying inorganic granular solids, so it quotes a low price. After talking to a couple other dryer manufacturers, even a few with different-style dryers, John decides RDC1 is the way to go. The dryer is installed, it works great, everyone is happy.
William is a friend of John’s, and he also has a problem. He needs to drop the moisture of a product by 40 points. When William hears how well John’s dryer works, he decides to call RDD1 for a quote.
William has another problem though, too. His product is different than John’s: It’s an organic material. When William tells the engineer at RDD1, he says that RDD1’s specialty is inorganic granular solids. But he’s pretty sure RDD1’s dryer can handle John’s product. He quotes the same system as they quoted John, a direct-fired system, with a low price.
Before purchasing a dryer, William wants to talk to a couple of other dryer companies. Rotary-Drum Dryer Two (RDD2) tells him that to dry this particular organic material, he will need an indirect dryer because of its delicate nature and susceptibility to damage at higher temperatures. RDD2 quotes him a higher price.
William wants more options, so he contacts Rotary-Drum Dryer 3 (RDD3). RDD3 can dry his product in a direct-fired rotary-drum dryer, but they quote an even higher price. The engineers at RDD3 specifying the system say that they regularly dry this material with great success. The also tell William that, due to the nature of his product to be dried, a normal direct-fired system might burn some of the product, but that RDD3 solved that issue years ago.
William stews over this information. He’s been told by two different dryer companies that they can dry his material with a direct-fired system, and RDD1 was much, much less expensive. Besides, John is super happy with his dryer! So William selects RDD1.
The system from RDD1 is installed. Everything seems to be going along well for several months, except William has a sneaking suspicion that he should be getting out more product than he is. And, there have been a few fires — but nothing too serious.
At the end of the year, John and William get together to chat about their dryers. John is super happy, his dryer is doing everything RDD1 said it would. He’s even been able to push his capacities up and get more throughput.
William hasn’t had this experience. He recently went through the product logs and calculated the amount of dry product he was expecting, based on infeed moisture content and rates, and outfeed moisture content and rates. He calculated that nearly 10 percent of his product just vanished. And, it seemed that every time he tried to push the dryer to the rated throughput, he would get a fire somewhere.
The conversation prompts John to check his numbers, but his output was what he expected. He gets some dust through his baghouse, but he included that in his calculations. William’s missing 10 percent was after he accounted for the fines coming through his baghouse, which had also caught fire several times throughout the year. William asks John if he has any trouble with fires, to which John looks at him, baffled, and says no, they haven’t had any issues.
Why the difference? Why would the same dryer work for John and not nearly as well for William? This article will examine some key differences between drying inorganic and organic material. In addition, it will explain about factors to consider — such as drying goals, safety risks, product handling, heat sensitivity and corrosiveness and abrasiveness — when researching drying technology.
Ultimately, a dryer system also functions as a product-conveyance method. Not only do different products require specialized drying environments but also specialized methods of conveyance, whether pneumatic, mechanical or both.
No matter what you are looking to dry, all buyers have a common goal: to remove water from a product without degrading the material.
Yet not all dryers are the same. Just because a system can effectively dry one product does not mean it can effectively dry all products. When you are in the market for a dryer, it is imperative to work with a dryer designer who understands your product to get the best end result.
Carbon-based — or organic — materials tend to be temperature sensitive, combustible and easily damaged by radiant heat. As OSHA has noted about combustible dusts, more than half of the list is composed of organic products. Certain chemical, metal or plastic dusts also present safety hazards that should be taken into account when purchasing or upgrading a drying system. Failure to do so can create an unsafe work environment.
Inorganic materials of the granular, solid variety are regularly dried in rotary-drum dryers. Such products are less vulnerable to fires and explosions because the product is typically not flammable. While inorganic materials pose a lower fire threat than organic materials, there are still safety factors that purchasers and designers should consider.
Prior to working with a dryer designer, review the safety data sheet (SDS) for your specific product. At normal drying temperatures, there is little chance of thermal degradation for the majority of inorganic granular solids, specifically of the mineral and salt variety. However, some materials are volatile enough to require indirect or low temperature drying. In such systems, the combustion gases or flame never contact the material. This minimizes the chance of a sparkler or other source of ignition contacting the product.
Ultimately, the rotary drum portion of your dryer is a product conveyor. Having flighting tailored to the properties of your product ensures that it is pneumatically or mechanically conveyed through the system for uniform drying — and with little material degradation.
While some materials are fairly straightforward and only require moisture reduction, other products go through a sticky phase. Distillers’ grains are one example of a product that requires product recycling. Improper handling of materials that have a sticky phase can result in overdried, charred product. Experienced dryer designers understand the complexity of drying products with these properties and design a system that prevents damaged or, in some cases, completely unusable product.
Another important consideration is heat sensitivity. High drying temperatures and radiant energy from open flames can cause problems for both organic and inorganic materials. (But, not always.) Some inorganic materials have specific temperatures that induce oxidation or thermal decomposition.
It is important to know these temperatures to prevent product damage. All organic materials inherently have energy content and an associated autoignition temperature. Some products are more delicate and temperature sensitive than others. These numbers can typically be found on safety data sheets (SDSs) as well as reference engineering websites.
Corrosiveness and Abrasiveness
To maximize the lifespan of your dryer system, your dryer designer must choose the best possible fabrication material given your desired temperatures, drying environment and product.
Mild carbon steel is affordable, durable and has good ductility characteristics that make it well suited to cyclic stresses; however, it cannot withstand corrosive or abrasive materials. Stainless steel 304 and 316 are popular choices to dry some corrosive products such as distillers’ grains. When making the switch from mild carbon steel to a stainless-steel alloy, ease of fabrication and high ductility are lost. Not only does it cost more, but many stainless varieties are far more susceptible to cyclic stresses, which could cause maintenance problems within a few years.
Highly abrasive materials may require abrasion resistant wear plating, but those materials cannot always handle high heat, impact or corrosion.
These are all details that need to be taken into account when researching a suitable drying system. Select a drying company that is familiar with the pros and cons of each metal type. Such a supplier can help you determine the materials that will work best with your product.
The internal flighting of this rotary drum facilitates even showering of the product through the gas stream while allowing resting spots to prevent overdrying. The material is pneumatically conveyed, concurrent with the gas flow, through this style of drum.
Outfeed Product Quality Matters
Your product matters, your bottom-line matters and you should not settle for a one-size-fits-all dryer. Working with a company that thoroughly understands the nuances of the material you are processing will product a better-quality end product, have a higher yield, and create a safer work environment.
Final Lessons from William and John
Given the key points outlined in the article and returning to our original scenario, can you guess what wasn’t working for William?
John’s product was not getting burned by the flame, so increasing his burner output increased his throughput.
William’s product was getting burned by the flame and damaged by the radiant energy. These factors account for the losses he discovered. When William tried to increase his burner output, more material was getting burned up and overdrying, causing fire hazards. This also increased the chance for a burning ember being carried over to the baghouse, where it would ignite the bags.
After his conversation with John, William researched dryer fires and learned that they are relatively common for the material he is drying. He also learned that the National Fire Protection Association will require a dust hazard analysis (DHA) beginning in 2020.
To address these concerns, William hires a reputable firm to help him with his DHA. He retrofits his dryer, at considerable expense, to conform to the suggestions laid out during the DHA. His dryer may not have been specifically designed to handle organic materials, but at least it will be much safer. Now, if only William could figure out how to keep that extra 10 percent from burning up, life would be good.